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A: I'd heard the same thing for decades, but I wasn't sure it was true. I had to do some research to answer your question.

Several recent studies have looked into this question. The results were not unanimous, but most studies show that eating late in the day does contribute to weight gain and other health problems.

In one study, researchers found that eating a big breakfast and a small dinner is better for weight loss than eating a small breakfast and a large dinner.

Ninety-three women with metabolic syndrome participated in this study. (Metabolic syndrome is a set of risk factors that increases your risk for heart disease, stroke, diabetes and other health problems.) The women were randomly assigned to different diets. Group 1 ate a 700-calorie breakfast, 500-calorie lunch and 200-calorie dinner. In contrast, Group 2 ate a 200-calorie breakfast, 500-calorie lunch and 700-calorie dinner.

After 12 weeks, the researchers measured weight loss in both groups. Women in Group 1 lost significantly more weight than women in Group 2. The women in Group 1 also had lower insulin, glucose and lipid levels. This shows that this eating pattern benefits overall health, as well as weight loss.

A problem with late-night eating is that most people tend to overeat at that time. We eat out of boredom, or fail to adjust our daytime calories to allow for a nighttime snack. Nighttime snackers are more likely to develop unhealthy eating habits, which can lead to weight gain.

Another factor that may come into play is the body's circadian rhythm, or sleep-wake cycle. One study suggested that our body processes food differently at different times of day. Another recent study, by researchers at Brigham and Women's Hospital and Harvard Medical School, found that the body's circadian rhythm makes us hungrier in the evening.

I'd bet the main reason that calories late in the day are burned less efficiently has to do with circadian rhythms. We 21st-century humans (the Homo sapiens species) and the Homo erectus and Homo ergaster species that preceded us go back 1-2 million years. Until the last few hundred years, all but the most affluent of us didn't have much light after the sun went down. That must have meant that we ate during daylight and went to sleep soon after sunset.

I assume that over those 1-2 million years our bodies adapted to that reality. Our bodies probably are built to burn energy most efficiently at the time of day - for most of our history - we ate.

Whatever the mechanism, eating late at night does appear to contribute to weight gain. If you're trying to lose weight, put an end to those late-night snacks.

DR. ANTHONY KOMAROFF is a physician and professor at Harvard Medical School. Go to his website to send questions and get additional information: www.askdoctork.com.

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